In addressing concerns about bioterrorism and bio-warfare there has been significant activity in recent years towards development of new sensors to detect the presence of pathogenic biological aerosols. How ever, the detection and positive identification of bio-agent particles in the air presents many challenges. Ideally, detection systems should be capable of rapidly detecting and confirming bio-agents at low concentrations with high detection (true positive) probability but simultaneously with low false positive probability. Therefore, detection assays should be sensitive and specific, capable of detecting low concentrations of target agents without interference from background materials. However, these requirements have proved difficult to achieve, employing biological-threat detection systems in efforts to process real world samples. In typical atmospheric environments, the number density of ambient aerosol particles frequently becomes large enough to easily mask the presence of a minor subpopulation of a biological aerosol at concentrations of concern. In collecting aerosol samples, the presence of ambient background particles creates a complex matrix material that can interfere with, or inhibit, biological detection/identification assays. There is overwhelming evidence that the sensitivity, speed of detection, and levels of misclassification of detection systems are compromised when the systems are presented with air samples containing dust from a variety of environments. The failure of polymerase chain reaction (PCR) analysis to correctly identify and quantify the number of targets for a given experiment has caused researchers to explore a variety of sample processing steps as a means to remove the inhibiting factors prior to running the identification assay. Such preprocessing includes sample dilution, DNA capture onto magnetic beads, and development and use of special reagents such as Gene Releaser. Each of these sample preparation methods has a significant cost associated with its use as well as adding time and complexity to the detection system.
Generic biological-threat detection systems employ modules and/or subsystems composed of concentrator subsystems, interrogator subsystems, and sample collector subsystems. These generic biological-threat detection systems are compatible and combinable with a wide variety of specific biological sample detection modules and/or subsystems.
Laser-induced fluorescence (LIF) in the ultraviolet (UV) spectral region has been used in front-end trigger sensor modules for detecting bio-threat agents. UV-LIF systems can be operated autonomously and continuously and are efficient in differentiating biological from non-biological particle compositions. However, discrimination among biological organisms has been typically limited to large classes (fungal/bacterial/viral, for ex ample) and has not shown sufficient specificity to reach the level of speciation. Modules, systems and/or subsystems employing multiple-wavelength excited fluorescence have shown improved discrimination capabilities compared to single-wavelength excitation systems. The feasibility of using fluorescence and scattering signatures to classify individual aerosol particles on-the-fly in order to separate and collect selected particles has been previously explored using a pulse, or jet, of air to deflect selected particles and collect them in a different spatial region, enabling separation and classification at a rate of only about 300 Hz.
The ability to accurately measure the velocity and track the position of individual particles for on-the-fly classification and selective capture is essential for next-generation chemical and biological agent detection systems. Velocity of a particle can be determined by measuring the transit time between two parallel light beams of known spacing by detecting the scattered light pulses with a photoelectric detector such as a photomultiplier tube (PMT). The accuracy of the measurement improves with increasing separation between the two beams until the particle throughput rate becomes so large that it becomes difficult to keep track of the individual particles. If two or more particles enter the measurement zone before the first particle has traveled through both beams, then it can be difficult to assign which light scattering event is associated with which particle. To track the particle and measure the velocity reliably we need to be able to detect the position of the particle and identify the pair of pulses that correspond to each particle.
One approach to overcome this problem is to use two separate beams from different laser sources, operating at two different wavelengths. In this way, the signals from the two beams can be easily distinguished. Problems with this approach include the mechanical stability of the optical mounts and the pointing stability of the two laser sources, any small change in the positions of the beams will result in error in the velocity measurement. The requirement of the use of two lasers also adds to the expense and complexity of the system.
The need exists for the ability to detect and classify individual aerosols in real time.
Furthermore, the need exists for the ability to enrich the concentration of suspected threat particles by sorting and collecting aerosols based on their classification, thus improving the reliability of the threat analysis and reduce the frequency of false alarms.
Further, the need exists for methods and systems that reduce or eliminate the problems involving particle through put rate becoming so large that it becomes difficult to assign which light scattering event is associated with which particle.
Further the need exists for the ability to detect the position of the particle and identify the pair of pulses that correspond to each particle.
In addition, the need exists for overcoming problems introduced by using two separate beams from different laser sources, such problems include problems of mechanical stability of the optical mounts and the pointing stability of the two laser sources.
Further, the need exists for reducing the expense and complexity of using multiple lasers in velocity measurements of particles.